1. Field of the Invention
The present invention relates to a fundus examination apparatus suitable for use in a diagnosis of glaucoma,more particularly, to a fundus examination apparatus capable of measuring thickness of a bundle of nerve fibers with high accuracy.
2. Related Art Statement
Conventionally, there has been known a fundus examination apparatus to be mainly used for an eye exam of glaucoma and the like (reference to, for example, Japanese Patent No. 3235853).
The above conventional fundus examination apparatus comprises an illumination optical system, which illuminates a fundus by fundus illumination light of circularly-polarized light, an imaging optical system, which photographs the fundus image of a bundle of nerve fibers of the fundus based on the polarization properties of the illumination light reflected from the fundus by this illumination, and picture signal processing means, which visualize the thickness distribution of the bundle of nerve fibers based on the image photographed by the imaging optical system with a plurality of polarized light states.
Moreover, in the above fundus examination apparatus, the imaging optical system detects the elliptically-polarized light of the fundus reflection light through a polarized light element, and the picture signal processing means obtain the phase distribution regarding the thickness of the bundle of nerve fibers based on the elliptically-polarized light to visualize the thickness distribution of the bundle of nerve fibers.
Furthermore, the imaging optical system includes a TV camera, and the picture signal processing means include a frame memory which stores the image photographed by the TV camera.
There has been also known a fundus examination apparatus capable of measuring thickness of a bundle of nerve fibers. The fundus examination apparatus comprises an illumination optical system, which illuminates a fundus of subject eye by circularly-polarized light, and an imaging optical system in which a CCD camera for imaging the fundus image based on the illumination light reflected from the fundus is disposed. In the above fundus examination apparatus, the imaging surface of the CCD camera is integrally provided with a polarized filter. The polarized filter comprises small plate portions, which are repeatedly disposed. Each of the small plate portions comprises micro polarizing plate portions for resolving the reflected illumination light into linearly-polarized light components, which are orthogonal to each other, and micro polarizing plate portions for resolving the reflected illumination light into linearly-polarized light components having directions intersect ant to both of the linearly-polarized light components (reference to, for example, JP-A-2001-137190). In addition, the micro polarizing plate portions are disposed adjacent to each other.
In this fundus examination apparatus, the micro polarizing plate portion corresponds to each pixel of the CCD camera, and the liner polarized light components having the directions intersect ant to both of the linearly-polarized light components are disposed in the direction at an angle of 45° with respect to both of the linearly-polarized light components.
Furthermore, it is not a fundus examination apparatus, but there has been known a polarized light measuring apparatus which measures polarization properties of an object (reference to, for example, JP-A-2002-116085).
The above polarized light measuring apparatus comprises a condensing optical system which condenses light from an object, a phase plate array in which an unit constitutional plate comprising four micro phase plates, which change the phase state of the light from the condensing optical system and have different angles of the advancing phase axes each other, is arranged in a matrix in a plane, one polarizer which polarizes the light passing through the phase plate array in a predetermined direction, a plurality of light receiving elements (image pick up device), which receives the reflected illumination light uniformed in one polarization direction by passing the polarizer, and arithmetic processing means which calculate Stokes' parameter regarding the light from the object based on the intensity of the reflected illumination light received by the plurality of light receiving elements, respectively.
By the way, in the fundus examination device, polarization properties (double reflection) exist in a bundle of nerve fibers, and reflected light having different velocity each other is generated by the double reflection generated when passing through the bundle of nerve fibers. Accordingly, the fundus examination apparatus utilizes a principle that the difference (phase difference (retardation)) of the velocity generated in the components of the reflected light correlate with the thickness of nerve fibers.
More particularly, as shown in FIG. 1, if illumination is carried out by using an illumination light P0 of circularly-polarized light, the illumination light P0 is reflected by a borderline F between a vitreous body I and a retina (a bundle of nerve fibers) D. Since the reflection by the borderline F is specular reflection, the reflected illumination light P1 maintains the circularly-polarized light.
Next, the illumination light P0 is reflected by a borderline G between the retina D and a pigmented layer G. However, the illumination light P0 becomes elliptically-polarized light by the difference of the velocity of the reflected illumination light P2 when passing through this bundle of nerve fibers. When a bundle of nerve fibers is completely defected, the reflected illumination light P2 maintains the circularly-polarized light, but the reflected illumination light P2 becomes the elliptically-polarized light as the thickness of the bundle of nerve fibers increases, and the oblateness is increased. If a circularly-polarizing plate having a direction same as the reflected illumination light P is provided in the light receiving system, the specular reflection light P1 by the borderline F between the vitreous body I and the retina D becomes the reversely oriented circularly-polarized light; thus, the specular reflection light P1 disappears (reference to Japanese Patent No. 3235853, paragraph [0012] [0013]).
Accordingly, the thickness of a bundle of nerve fibers can be measured by the oblateness of the reflected illumination light P2 of the elliptically-polarized light.
However, the illumination light P may enter into the inside of the pigmented layer G, and may be scattered and reflected by the inside of the pigmented layer G. Therefore, the illumination light P3 scattered and reflected by the inside of the pigmented layer G becomes a non-polarized light state.
The scattered and reflected illumination light P3 is directed to the imaging optical system with the non-polarized state, so the light volume of the scattered and reflected light P3 from the inside of the pigmented layer G is received by the imaging optical system.
Therefore, the light volume of the reflected illumination light P received by the imaging optical system includes the light volume of the scattered and reflected illumination light P3 of non-polarized light from the inside of the pigmented layer G as noise. The light volume of the scattered and reflected illumination light P3 shown in FIG. 1 is added to the reflected illumination light P of elliptically-polarized light shown in FIG. 2 as a noise component, and the reflected illumination light of partially-polarized light enters into the imaging optical system. Consequently, an apparent error is included in the actual thickness of a bundle of nerve fibers.